Staff of the Siding Spring Observatory in NSW's northwest are among those who lost homes in the most destructive bushfire in the state for a decade.

The fire swept by the hilltop observatory near Coonabarabran on Sunday but it escaped largely unscathed, though a lodge, a cottage and a storage building were destroyed and other buildings damaged.

The Australian National University, which runs the site, said on Tuesday its priority now was the safety and wellbeing of staff and their families, a number of whom lost their homes in the fire.

The fire destroyed at least 33 homes, along with farm sheds, livestock, machinery and more than 40,000 hectares of land as it burnt in and around the Warrumbungle National Park.

It has forced the closure of the observatory for two weeks for damage assessments and to ensure the site is safe for staff to return.

An ANU team including Vice-Chancellor Ian Young, Research School of Astronomy and Astrophysics director Mathew Colless and a staff counsellor will visit Coonabarabran on Wednesday to meet staff and their families.

The ANU said the needs of those affected was being assessed and it was looking at launching an emergency appeal so ANU members and the wider community could support them.

"Fire preparation works appear to have been successful in preventing catastrophic damage to the scientific facilities," the ANU said in a statement.

It said it had about $80 million worth of assets at Siding Spring, which were fully covered by insurance.

More than 80 Rural Fire Service volunteers supported by 18 aircraft spent most of Monday trying to contain the 100-kilometre wide front that burned through the area near Coonabarabran.

About 100 people living in the area were forced to evacuate their homes and the RFS on Monday night said the blaze remained ''a large and dangerous bushfire'' that was worse than the Black Christmas fires in 2001.

On Monday night there were 125 bushfires burning in NSW, 30 of which were uncontained.

The acting Premier, Andrew Stoner, described the bushfire season so far as ''a hell of a week'' but while milder temperatures helped firefighters on Monday, winds and temperatures were expected to increase by Friday.

The Bureau of Meteorology said most of the state would be in the high 20s to mid-30s with Sydney temperatures rising from 30 degrees on Wednesday to 37 on Friday. Bourke is forecast to be in the high 30s for most of the week, peaking at 40 degrees on Saturday.

As Australia recovers from last week's record-breaking temperatures, the head of the United Nations Intergovernmental Panel on Climate Change, Rajendra Pachauri, said it was clear heatwaves were occurring more frequently, and would increase further as the planet warms.

''It [last week's heatwave] could be [a result of climate change], but I wouldn't draw any conclusions on one single event,'' Dr Pachauri said. ''I think you have to take the whole aggregation over a period of time and then come up with the conclusion, which is precisely what we have done.

''They [the findings] are very, very clear. Heatwaves are on the increase, extreme precipitation events are on the increase, and on that there is really no room for doubt any more.''

An RFS spokesman said the Warrumbungle National Park fire had been ''absolutely shocking''.

''At one point there was a smoke plume rising 14 kilometres in the air. The fire was so big and there was so much smoke, it was shocking,'' he said.

''It's still not under control. It's still burning in the Bugaldie area. It's been a big effort to get around it.

''The winds, the temperature, the low humidity, just shocking.''

A teacher at Coonabarabran's high school, Peter Morrissey, nearly lost the family home in the Yerrinan Valley.

''We're very lucky, but unfortunately that's not the case for everyone,'' he said. ''The home just next door has been burned to the ground, while others have remained untouched.''

Firefighters were able to establish containment lines on a fire about 20 kilometres east of Cooma, in the state's south. The fire burned through more than 12,000 hectares of bush and grassland.

RFS volunteers have worked for a week now, fighting more than 100 fires across the state.